In order to obtain television images on a large screen, one prior art method picks up a television picture on a relatively smaller cathode-ray tube and projects a magnified picture of the television picture on the screen by the use of a projection lens. Since it is desired that the projected picture be as bright as possible, a lens having a large aperture ratio and a highly efficient screen are used, and in addition the input power is increased to cause the projection tube to emit light with a high brightness. However, there is a danger of implosion of the projection tube due to thermal expansion. Hence, a method has been proposed which cools the face plate of the projection tube with a liquid so as to avoid an implosion.
Referring to FIG. 1, an example of a prior art liquid-cooled projection tube is shown, in which a windowed metal plate 3 is fixed by an adhesive 4 to a face plate 2 of a projection tube 1 and a flat glass plate 5 is fixed by an adhesive 6 to the metal plate 3. A liquid 7 fills a sealed space defined in the window, so that heat generated from the face plate 2 is transferred to the metal plate 3 by convection from the liquid 7 and then is dissipated from the surface of the metal plate 3 into the surrounding atmosphere. This construction can reduce the temperature at the face plate 2, thereby preventing implosion of the projection tube. Also, the input power to the tube can be increased by an amount equivalent to the reduced temperature to thereby enable the projection tube 1 to emit light having high brightness. The flat glass plate 5, when made of X ray absorbing glass, can absorb the X rays emitted by the projection tube.
The liquid-cooled projection tube shown in FIG. 1 should not leak the liquid. Therefore, the bonding work must be carried out carefully, and therefore the bonding work takes much time. It is proposed for facilitating the bonding work to mount a holder which is an aluminum die casting on the tube 2 in a manner so as to enclose the flat glass plate 5 and the face plate 2, but this requires complicated parts which results in a high manufacturing cost. In any case, the assembly and parts are expensive.